By far the largest sources of potassium salts are marine brines and the evaporite salts formed by the evaporation of marine brines. These brines are found worldwide and have been formed under various conditions so their compositions are quite varied. Most potassium salts are recovered as potassium chloride since this compound is frequently found unaccompanied by more than one or two other salts in major proportions. In such cases, the separation of a commercially pure potassium chloride is relatively inexpensive. However, potassium salts are widely used as a fertilizer and for this use there is a strong preference for potassium sulfate over potassium chloride, especially with crops such as citrus fruits and tobacco. In fact, the demand is great enough that potassium sulfate is frequently manufactured from potassium chloride at considerable expense.
Potassium sulfate can be recovered also from many natural deposits of salts or from many salt mixtures obtained by evaporation of natural brines. However, potassium sulfate production from such sources is usually complex because of the presence of many other salts in significant proportions. Indeed, in such salt mixtures the potassium is almost always present chemically combined with another salt in a "double salt;" for instance, as langbeinite, kainite or glaserite, which salts are representative of a large group of double salts containing potassium. Many schemes have been presented for separating potassium sulfate from various mixtures of these salts. One type of separation process depends on solubility relationships which are complex and generally give poor yields. Another group of separation processes may be termed mechanical; flotation and tabling are examples. In the case of potassium sulfate, a mechanical type of separation step is frequently combined with a solubility process step in order to make the latter more efficient. Mechanical methods of separation usually have reasonably good yields. However, they usually require grinding the salt mixture and inasmuch as the salt mixture frequently must be ground extremely fine, the grinding step becomes quite expensive.
So, in general, it may be said that there are many kinds of deposits from which potassium sulfate may be recovered; and there are many known processes or combinations of processes which can be used in such recovery. However, all such processes that have been tried commercially have significant drawbacks. It is desirable, therefore, to provide a simple and economical process for separating certain double salts containing potassium from other salts occurring in brine evaporites; and to use this novel separation process in an improved process for recovering potassium sulfate from mixed salts formed by solar evaporation of marine brines.